This invention relates to a method of operating a tandem mass spectrometer to improve signal-to-noise ratio of an ion beam. The invention has particular, but not exclusive, application to triple quadrupole mass spectrometers using electrospray ionization techniques.
Tandem mass spectrometry is widely used for trace analysis and for the determination of ion structure. Commonly, the mass spectrometers used are quadrupole mass spectrometers which each have a set of four elongated conducting rods. In particular, triple quadrupole systems are widely used for tandem mass spectrometry. During operation, the mass resolving quadrupoles at either end of the triple quadrupole arrangement, are pumped to a relatively high vacuum (10xe2x88x925 Torr) while a central quadrupole is usually located in a collision cell and is maintained at a higher pressure for the purpose of promoting fragmentation of selected precursor ions.
Conventional resolving quadrupole mass spectrometers are subjected to both RF and DC voltages that require stringent length and machining requirements on the rod set. For instance, these rods are made of metallized ceramic, have a length of 20 cm or more and roundness tolerances better than 20 micro-inches and straightness tolerances better than 100 micro-inches. However, quadrupoles can also be operated in a condition where they are only subjected to RF voltages. In this case, the length limitation characteristic of RF/DC resolving quadrupoles no longer applies (rods as short as 2.4 cm may be used) and mechanical tolerances for rod roundness and straightness are considerably relaxed (tolerances of +/xe2x88x92{fraction (2/1000)} of an inch are used). Furthermore, there is no need for high precision, high voltage DC power supplies in the RF-only mode of operation.
When both DC and RF voltages are applied between the rod sets of the quadrupole, the quadrupole acts as a mass filter such that only ions of a pre-selected mass-to-charge ratio can pass therethrough for detection by an ion detector. The RF and DC voltages are varied depending on the frequency of operation and the mass range of interest. In the case of applying only an RF voltage to the quadrupole, the quadrupole acts as an ion pipe, transmitting ions over a wide mass-to-charge ratio while also permitting gas therein to be pumped away. Mass resolution can also occur in RF only quadrupoles since ions that are only marginally stable under a particular applied RF voltage gain excess axial kinetic energy due to the exit fringing field of the rod structure.
The structure and operation of a typical tandem mass spectrometer will now be described including commonly accepted designators for individual rod sets. Firstly, ions are produced from a trace substance that needs to be analyzed. These ions are guided and focused via an RF-only (typically 1 MHz) quadrupole rod set (Q0) to a first mass spectrometer including a quadrupole rod set (Q1), acting as a mass filter, for selecting parent or precursor ions of a particular mass-to-charge ratio. These selected precursor ions are then sent to another rod set (Q2) that has collision gas supplied to it thus acting as a collision cell for the fragmentation of the selected precursor ions. Typically, a collision cell is only subjected to RF voltage. The fragment ions are then sent to a second mass analyzing quadrupole rod set (Q3) that acts as a scannable mass filter for the daughter or fragment ions produced in the collision cell. A detector detects the ions selected in the second mass analyzing quadrupole, for recordal to generate a spectrum of the fragment ions. In tandem mass spectrometers, the gases used in the focusing rod set and the collision cell improve the sensitivity and mass resolution by a process known as collisional focusing (U.S. Pat. No. 4,963,736).
Unfortunately, known ion sources do not generate a pure stream of ions. Thus, mass spectra obtained from ions generated by atmospheric pressure ionization techniques such as electrospray ionization frequently contain many unwanted chemical components. These components are often due to cluster ion formation in the atmosphere-to-vacuum interface, the presence of which impedes identification of target analytes. In addition, there is sample dependent background noise from high velocity ions and clusters from the RF-only mass spectrometer. However, the inventor of the present invention has found that many of these unwanted cluster species are more fragile than the target analytes and can thus be discriminated against with the use of ion fragmentation techniques. This will allow for preferential detection of precursor ions.
In accordance with the present invention, there is provided a method of improving the signal to noise ratio of an ion beam, the method comprising:
(1) subjecting the ion beam to a first mass resolving step, to select precursor ions;
(2) colliding said precursor ions with a gas, to promote at least one of fragmentation and reaction of unwanted ions, whereby the unwanted ions generate secondary ions having a mass-to-charge ratio different from the mass-to-charge ratio of the precursor ions; and
(3) subjecting the ion beam including the secondary ions to a second mass resolving step, to reject ions with a mass-to-charge ratio different from the mass-to-charge ratio of the precursor ions, thereby increasing the signal-to-noise ratio of the ion beam.
Preferably the method includes effecting step (1) in a first mass spectrometer, step (2) in a collision cell, and step (3) in a second mass spectrometer. More preferably, the method includes scanning the first mass spectrometer through a range of mass-to-charge ratios and synchronously scanning the second mass spectrometer to select ions with the mass-to-charge ratio of the precursor ions. Alternatively, step (3) can be effected in a collision cell.
Depending on where step (3) is effected, the second mass spectrometer or the collision cell can either be operated to reject ions having a mass-to-charge ratio less than the mass-to-charge ratio of the precursor ions, or can be set to reject ions with mass-to-charge ratios both greater than and less than the mass-to-charge ratio of the precursor ions.
Preferably, the first and second mass spectrometers are quadrupole mass filters and the collision cell includes a quadrupole rod set. Further, the first and second mass spectrometers can be either one of a 3-dimensional ion trap mass spectrometer, a 2-dimensional ion trap mass spectrometer or a time-of-flight mass spectrometer. In addition, the second mass spectrometer can be provided as a quadrupole operated in RF-only mode with a q value between 0.6 and 0.907.
The collision cell can include an RF quadrupole or multipole having RF voltage applied to it which can be adjusted such that the precursor ions of interest emerging from the first mass spectrometer are transmitted to the second mass spectrometer. This collision cell contains neutral gas to promote collisional activation and subsequent fragmentation of the unwanted ions.
An alternative method would be to apply a resolving DC voltage to the second mass spectrometer while maintaining a q value near 0.706. This resolving DC voltage enhances the selectivity of the precursor ions over the unwanted ions.
As noted above, another alternative method would be to operate the collision cell with a and q parameters such that only the precursor ions of interest are stable and thus transmitted to the ion detector. This avoids the need for a second mass spectrometer.
Thus, this method increases the signal-to-noise ratio of an ion beam containing an analyte ion species with fragmentation thresholds greater than unwanted chemical species in the ion beam such as clusters that are more fragile than the analytes of interest. This results in considerable spectral simplification and easier identification of the analyte ions of interest. The ion beam can then be subject to further steps of fragmentation and/or reaction by mass analysis, in known manner.
Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings.